The discovery of enhanced separation of oil-water emulsions and dispersions using microwave radiation was first disclosed in 1986 by N. O. Wolf in U.S. Pat. No. 4,582,629. In this disclosure, Wolf demonstrated through several benchtop experiments that modest amounts of microwave power applied to oil-water emulsions could increase oil-water separation rates by more than a factor of two compared to simple heating alone. Results suggested that microwaves were enhancing the separation rate through a mechanism distinct from heating alone. Wolf postulated that microwaves were successful because of direct heating of the bulk of the emulsion and disruption of surfactant molecules present in the interfacial film.
Since Wolf's pioneering work, independent confirmation of his general results were obtained by several researchers. For example, D. A. Purta with the support of the EPRI Center for Materials Production developed a novel apparatus for testing continuous separation of emulsions using microwave radiation. Purta found significant reduction in the time required to separate oil and water phases of emulsions using only small amounts of microwave energy and with temperature rises of only 20 degrees C.
A research report, prepared by C. S. Fang, B. Chang, P. Lai, and W. J. Klaila, presented systematic data on the effectiveness of microwave radiation in separating water-oil mixtures and emulsified oil-water-solid sludges. The authors concluded that microwave radiation was more effective in heating thick, viscous emulsions than gas or oil-fired heaters. Enhanced emulsion breaking with microwave radiation was also reported. They found evidence that enhanced separation rates were due to reduction of the zeta potential, which suspends water droplets and solid particles in an emulsion.
Following the original disclosure of Wolf, a comprehensive series of patents were generated which disclosed novel concepts for applying microwaves to oil-water emulsions. Among these are U.S. Pat. Nos. 4,853,507; 5,055,180; 4,810,375; and 4,853,119. All of these patents assert the advantages of enhanced emulsion breaking properties through the application of microwave radiation. While generally applicable to any type of emulsion or suspension, one of the largest potential users of microwave-enhanced emulsion breaking technologies is the petroleum industry. Most of the patents referenced previously discuss applications in this specific industry.
In the petroleum industry, crude oil pumped from wells is typically co-mingled with suspended solids and water. Since the water and solids present problems if contained in refinery feedstocks, it is necessary to remove these components. The separation of oil from water and solids using gravitational settling methods is typically incomplete. The unseparated left-over materials are waste products consisting of stable oil/water emulsions mixed with solid minerals.
These crude-oil by-products are generated in large quantities. It has been estimated that more than 2% of the crude oil currently pumped from the ground takes the form of these stable oil-water emulsions mixed with solids, forming crude oil sludges. Having no value to oil producers, the sludges are ejected into open pits and ponds or are left temporarily in large crude oil storage tanks. The sludges are presenting an ever-worsening remediation problem to oil producers and refiners.
Traditional methods to separate oil/water emulsions include application of heat, microbial breakdown, centrifugation, and chemical addition. Most of these methods do not recover marketable product. They provide only partial separation and typically leave large amounts of waste that must be carefully disposed of in accordance with strict government regulations. Heating is usually carried out by gas or oil-fired heaters. Conventional heating methods have problems with slow heat transfer into thick sludges, accumulation of heavy layers of solid residue on heat transfer surfaces, and loss of valuable volatiles. Chemical demulsifiers, such as alum and polyamines, are available to break oil-water emulsions, but these chemicals are subject to very restrictive regulations on discharge to public water. In addition, chemical treatment can be a relatively slow process that does not provide high levels of separation of some emulsions.
Microwaves work very effectively in crude-oil sludge separation, since microwaves penetrate deeply into the interior of thick sludges without heated-surface contact, providing a very effective heating alternative, irrespective of enhanced separation mechanisms. As the sludge is heated, viscosity is lowered, and rapid coalescence of liquid phases takes place. Once the liquid phases coalesce, separation occurs through natural gravity or through centrifugation. No chemicals are needed to force separation.
Considerable effort has been devoted to field-testing of pilot systems to process these crude-oil sludges using microwave energy. In one field-test, results were published in which 98-99% of hydrocarbons were recovered from sludge. The recovered oil can be sold as high-quality crude which can be refined without further treatment. The microwave separation process is also cost-effective. One company estimated that the microwave separation process, when fully developed, will cost less than half of the price of other sludge-separation processes.
One of the principal obstacles to commercialization of the technology has been the need for major innovations in the microwave applicator, which couples microwave power into the emulsions. Current systems experience major difficulties with microwave component failure and power coupling inefficiencies. The most widely used approach to applying microwave radiation to oil-water emulsions involves formation of various tapered or conical dielectric barriers which direct and shape the flow of the emulsions. The various shapes are intended to create gradually changing conditions for microwave energy flux in order to minimize microwave reflections from the emulsions and the dielectric barriers, and maximize microwave power absorption into the emulsions.
A dielectric absorber which produces a gradually changing environment for microwaves over distances greater than or on the order of a wavelength in the direction of microwave propagation will be referred to throughout this disclosure as a microwave beach. The gradually changing environment for microwaves along a microwave beach is analogous to the gradually changing depth of water as ocean waves approach an ocean beach. Just as ocean waves are effectively absorbed along an ocean beach because of the gradual change in water depth, so are microwaves effectively absorbed along a microwave beach because of the gradual change in environment presented to the microwaves.
The effective manner in which microwaves are absorbed in a microwave beach is well known in the microwave industry. Tapered and conical beach configurations, utilizing water as the absorptive medium, form low-reflectance terminations or dummy loads in many high-power microwave systems. In microwave transmission applications, gradual transitions are often used to reduce or eliminate microwave reflections between differing waveguide elements. These gradual transitions share with microwave beaches the feature of a gradually changing microwave environment that produces low microwave reflections.
Several major difficulties have been encountered in the adaptation of microwave beach configurations to crude-oil emulsions. First of all, the real part of the relative dielectric permittivity of dehydrated crude oil can be as little as 2 or 3 and the imaginary part of the permittivity can be less than 0.05. Microwave beaches in this situation are very long. While the dielectric constants of emulsions which have a high water content may be much higher, and the beaches much shorter, designs must account for the extreme cases to be effective in all situations. The long dielectric barriers required in extreme-case designs become very difficult to manufacture and considerable space is required to contain the long devices. This is not an issue when pure water is the absorptive medium, since water has a real dielectric permittivity of nearly 80 and an imaginary permittivity of 1-12, depending upon the frequency.
Attempts have been made to produce imperfect short beaches, which lower reflections to some extent. The remaining reflections are then further reduced with stub tuners. These designs have not been entirely satisfactory. Tuning in this situation can be difficult, because of the highly-variable dielectric properties of many emulsions, including crude-oil emulsions. The properties may change on a rapid time scale as new material is introduced into the system under high-flow-rate conditions. Dielectric properties may also change as the emulsion separates, and as the emulsion is heated. Rapidly changing dielectric properties necessitate rapid tuning adjustments, but these adjustments are difficult to make using relatively slow mechanical tuning mechanisms. In extreme situations, the variations in dielectric properties of the emulsions may even induce frequency variations in the microwave source if a magnetron oscillator is used. This frequency variation within the microwave source makes satisfactory tuning virtually impossible.
Finally, beach designs have required a dielectric barrier to contain and shape the emulsion. The presence of the barrier has led to numerous problems. For example, in short-beach configurations, portions of the barrier have been rapidly coated with thick residues from the microwave-heated emulsions. The presence of these residues is indicative of localized overheating of the emulsion and strong adherence of sludge components. These residues can be strong absorbers of microwave power, leading to strong localized heating of the barrier and further localized heating of regions of the emulsion in contact with the heated residues. Numerous barrier failures have been attributed to thermal stresses resulting from this strong localized heating brought about by microwave absorption in the barrier coating.
We disclose here a novel microwave applicator for emulsion breaking that overcomes these technical difficulties, and is capable of directly processing sludges and emulsions having a wide range of viscosities and material composition. Our disclosed solution will provide an attractive alternative to current methods, bolstering commercialization efforts and promoting widespread use of microwave emulsion breaking technologies in areas such as crude-oil sludge remediation and oil recovery. In these areas, the value of our disclosed technology is considerable in terms of environmental impact alone. In addition, the recovery of considerable quantities of useable oil from crude-oil sludges will have far-reaching economic benefits.